The Use of Semiquantitative Food Frequency Questionnaires to Estimate the Distribution of Usual Intake

We consider whether semiquantitative food frequency questionnaires can be used to survey a population so as to estimate the distribution of usual intake. We take as an assumption that if they were possible to obtain, the mean of many food records or recalls would be an accurate representation of an individual's usual diet. We then assume that nutrient intake as measured by a questionnaire follows a linear regression model when regressed against usual intake of that nutrient. If the coeecients in this regression relationship were known, then the distribution of usual intake could be constructed from the responses to the questionnaire. Since we generally do not know the values of the coeecients, they need to be estimated from a calibration study in which respondents complete the questionnaire together with multiple food records or recalls. This can be done either through an internal subset of the data, or through an independent external study. With an internal substudy, we nd that food frequency questionnaires typically provide little information about the distribution of usual intake, additional to that obtained from the multiple records or recalls in the substudy. The same is true for small external studies. However, with a large external study which accurately characterizes the relationship between the questionnaire response and usual intake, food frequency questionnaires can provide considerable information about the distribution of usual intake , especially when cost considerations are taken into account. The results do not apply to the diierent problem of correcting relative risks for the eeects of measurement error. 1